In DEP (Direct Electrostatic Printing) the toner or developing material is deposited directly in an image-wise way on a receiving substrate, the latter not bearing any image-wise latent electrostatic image. In the case that the substrate is an intermediate endless flexible belt (e.g. aluminium, polyimide etc.), the image-wise deposited toner must be transferred onto another final substrate. If, however, the toner is deposited directly on the final receiving substrate, a possibility is fulfilled to create directly the image on the final receiving substrate, e.g. plain paper, transparency, etc. This deposition step is followed by a final fusing step.
This makes the method different from classical electrography, in which a latent electrostatic image on a charge retentive surface is developed by a suitable material to make the latent image visible. Further on, either the powder image is fused directly to said charge retentive surface, which then results in a direct electrographic print, or the powder image is subsequently transferred to the final substrate and then fused to that medium. The latter process results in an indirect electrographic print. The final substrate may be a transparent medium, opaque polymeric film, paper, etc.
DEP is also markedly different from electrophotography in which an additional step and additional member is introduced to create the latent electrostatic image. More specifically, a photoconductor is used and a charging/exposure cycle is necessary.
Direct electrostatic printing is also quite different from ionography where an electrostatic latent image is formed on a charge retentive surface either by image-wise applying charges (ions) on that surface, or by image-wise neutralising charges on a uniformly charged charge retentive surface by image-wise discharging the surface by applying charges of different polarity (ions of different polarity). This latent image is then, as in classical electrophotography, developed by charged toner particles.
A DEP device is disclosed in, e.g., U.S. Pat. No. 3,689,935. This document discloses an electrostatic line printer having a multi-layered particle modulator or printhead structure comprising:
a layer of insulating material, called isolation layer PA1 a shield electrode consisting of a continuous layer of conductive material on one side of the isolation layer; PA1 a plurality of control electrodes formed by a segmented layer of conductive material on the other side of the isolation layer and PA1 at least one row of apertures. PA1 a toner delivery means, having a surface whereon charged toner particles are present for providing a flow of said toner particles from said surface to said substrate, PA1 a printhead structure with printing apertures and control electrodes, interposed in said flow of toner particles for image- wise controlling said flow, wherein: PA1 i) said toner delivery means comprises a number of n toner applicator modules, each having a width, WTD, smaller than said printing width PW, PA1 ii) said number n of said toner applicator modules is equal to or larger than 2, and PA1 iii) at least two of said number n of toner applicator modules are positioned in a staggered configuration with respect to said substrate.
Each control electrode is formed around one aperture and is isolated from each other control electrode.
Selected potentials are applied to each of the control electrodes while a fixed potential is applied to the shield electrode. An overall applied propulsion field between a toner delivery means and a receiving member support projects charged toner particles through a row of apertures of the printhead structure. The intensity of the particle stream is modulated according to the pattern of potentials applied to the control electrodes. The modulated stream of charged particles impinges upon a receiving member substrate, interposed in the modulated particle stream. The receiving member substrate is transported in a direction perpendicular to the printhead structure, to provide a line-by-line scan printing. The shield electrode may face the toner delivery means and the control electrode may face the receiving member substrate. A DC field is applied between the printhead structure and a single back electrode on the receiving member support. The propulsion field is responsible for the attraction of toner to the receiving member substrate that is placed between the printhead structure and the back electrode. The printing device as described in U.S. Pat. No. 3,689,935 is very sensitive to changes in distances from the toner application module towards said shield electrode, leading to changes in image density. For that reason it is very difficult to construct a printer for large format printouts.
Multi-applicator module printing systems have been disclosed, but only with the construction of different application modules perpendicular in the printing direction, leading to the possibility of obtaining a single pass multi-colour printer. Such descriptions have been given in e.g. U.S. Pat. No. 5,132,708, U.S. Pat. No. 5,283,594 and U.S. Pat. No. 5,477,250.
The teachings of these disclosures however, do not give a solution to the problem of printing large format images with sufficient image quality and printing speed.
There is thus still a need for a DEP printing system yielding reliable and stable images of large image size with a fast printing speed.